Method and assembly for improving the dynamic behavior of a coal-fired power plant

ABSTRACT

The invention relates to a method for improving the dynamic behavior of a coal-fired power plant for primary and/or secondary requirements of the power grid operator with respect to the current output into the grid, wherein the power plant has a nominal output (RC) and is operated by way of firing, wherein upon an increase in the primary and/or secondary requirements of the power grid operator with respect to the current output into the grid the coal dust volume that is supplied is raised with respect to the present actual output, and wherein upon a decrease in the primary and/or secondary requirements of the power grid operator with respect to the current output into the grid the coal dust volume that is supplied is lowered with respect to the present actual output and is stored, and to an assembly for carrying out the method.

Method and assembly for improving the dynamic behavior of a coal-firedpower plant for primary and/or secondary requirements of the power gridoperator with respect to the current output into the grid.

The invention relates to a method and assembly for improving the dynamicbehavior of a coal-fired power plant for primary and/or secondaryrequirements of the power grid operator with respect to current outputinto the grid.

Keeping the alternating voltage frequency in power grids constantconstitutes an important objective. Deviations from the predeterminedfrequency can result in the failure of consumers connected to the gridand consequential damages resulting from such failure.

Deviations from the predetermined grid frequency value mainly occur whenthe power requirement on the power plants connected to the power gridsuddenly changes because for instance a power plant is disconnected fromthe grid because of an accident or a large consumer is connected to thegrid or because the grid configuration or grid distribution changes. Inorder to keep the grid frequency constant at the predetermined value orwithin a certain tolerance range it has to be ensured, within the scopeof the so-called primary control or primary control output, that thegenerated power and the grid load remain balanced and as much electricpower is always generated as is consumed by the grid load when operatingwith a predetermined grid frequency. In doing so, the primary control isadditionally supported by the secondary control or secondary controloutput, which following the balancing of a sudden change of the consumedor the generated power through the primary control offsetsquasi-stationary deviations both of the frequency as well as of thetransfer power.

In order to be able to counteract deviations from the predetermined gridfrequency value in the shortest time, some national grid operatorsstipulate in their standards conditions or targets under which this hasto be accomplished. Thus, the British Grid Operator National GridElectricity Transmission plc for example through its document “The GridCode”, Issue 3, prescribes that in the event of a frequency deviation apower plant linked to the power grid, for example at an operating modeof 65% of its nominal output the power plant output within the scope ofthe primary control or the primary requirements is increased by 10% ofits nominal output, within 10 seconds, thus counteracting the frequencydeviation. This, in terms of time, very rapid and with respect to thepower output very large change makes major demands on the power plant,particularly on a coal-fired power plant.

As a rule, large coal-fired power plants are designed with coal dustfurnaces, with which the coal ground in the coal grinding plant can bedirectly fed to the firing box of the power plant via coal dust lines(so-called “direct” coal dust furnaces). The condition of the fuel isone of the main factors for good combustion, a sound efficiency, lowemissions and little that is uncombusted in the ash in order to be ableto utilize this by-product. For coal conditioning, the coal grindingplant or coal mill has to be in a stationary heat and mass flowequilibrium, which results in load changes on the coal dust furnace andthus on the power plant itself being able to be carried out only slowlyand thus a delay time occurring when load changes are carried out or arerequired.

The delay time of the coal mill with changing fuel quantity or charge isa substantial part of the overall plant delay time. The delay time ofthe coal mill can be long depending on the raw coal conditioning process(dependent on fineness, moisture, hardness of the raw coal and the millloading) and therefore has a detrimental effect on the delay time of theoverall plant.

The object of the invention now is to create a method for improving thedynamic behavior of a coal-fired power plant for primary and/orsecondary requirements of the power grid operator with respect to thecurrent output into the grid, with which the delay time of the coal dustfurnace of the power plant is reduced so that the power plant meets thetargets or conditions of respective national operators of power grids.It is, moreover, an object of the invention to create an assembly forimproving the dynamic behavior of a coal-fired power plant for primaryand/or secondary requirements of the power grid operator with respect tothe current output into the grid.

The object mentioned above is solved through the totality of thefeatures of Patent Claim 1 with respect to the method and through thetotality of the features of Patent Claim 9 with respect to the assembly.

Advantageous configurations of the invention can be taken from thesubclaims.

Through the solution according to the invention a method and an assemblyfor improving the dynamic behavior of a coal-fired power plant forprimary and/or secondary requirements of the power grid operator withrespect to the current output into the grid is created, which has orhave the following advantages:

-   -   Creation of the possibility for power plant operators to obtain        the required permits for building and operating power plants in        agreement with the prescribed national grid frequency        requirements.    -   Through the sale of primary control reserve, the power plant        operator is enabled to operate the plant more economically or        achieve higher profits.    -   The manufacturer or supplier of such power plants is enabled to        offer or sell these power plants on world-wide markets, e.g. UK,        Ireland, France, China, India, Singapore etc.

An advantageous embodiment of the invention provides that the silohaving a storage volume V_(Sp) in normal operation of the indirectfiring system is filled, in terms of volume, approximately half withcoal dust for storage and use upon increase of the primary and/orsecondary requirements of the power grid operator with respect to thecurrent output into the grid and the remaining storage volume is usedfor receiving and storing the excess-produced coal dust upon reductionof the primary and/or secondary requirements of the power grid operatorwith respect to the current output into the grid.

In an advantageous configuration of the invention, the increase orreduction of the indirectly fed-in coal dust quantity is effectedthrough a controlled increase or reduction of the throughput rate of theapportioning organs. Thus, the needs or the dynamic behavior of thecoal-fired power plant can be accurately taken into account.

An advantageous configuration provides increasing or reducing thevolumetric flow of the conveying gas blower in a controlled manner uponan increase or a reduction of the indirectly fed-in coal dust quantity.Thus, the smooth input of the coal dust in the firing box is maintained.

It is advantageous that the increase or the reduction of the throughputrate of the apportioning organs and/or the increase or the reduction ofthe volumetric flow of the conveying gas blower is brought about by theblock output control of the coal-fired power plant influenced by therequirements of the power grid. Through this measure it is ensured thatin the event of a frequency change or a requirement in the power grid aninfluencing signal of the grid control is directly sent to the blockoutput control of the coal-fired power plant and its furnace and acountermeasure without loss of time is thus initiated in order tooptimize the dynamic behavior of the power plant.

In an advantageous embodiment of the invention, the primary requirementor the primary control is triggered through a remote-controlled signal.In a further advantageous embodiment of the invention the secondaryrequirement or the secondary control is likewise triggered through aremote-controlled signal.

The secondary requirement or the secondary control can be additionallytriggered through written or oral instruction to the operating personnelof the power plant.

In the following, exemplary embodiments of the invention are explainedin more detail by means of the drawings and the description.

FIG. 1 shows an extract from the British Electricity Grid Regulations(Grid Code (UK)), wherein the extract shows the minimum requirementprofile of the frequency dependency for a 0.5 Hz frequency change fromthe target frequency (minimum frequency response requirement profile fora 0.5 Hz frequency change from target frequency),

FIG. 2 shows an extract from the British Power Grid Regulations (GridCode (UK)), wherein the extract shows the interpretation of the primaryand secondary control or primary and secondary requirement(interpretation of primary and secondary response values),

FIG. 3 shows, represented schematically, an assembly for improving thedynamic behavior of a coal-fired power plant for primary and/orsecondary requirements of the power grid operator for the current outputinto the grid, wherein the coal grinding plant including coal dust linesof the furnace of the power plant are shown,

FIG. 4 shows, represented schematically, the relation of an outputincrease as a function of time and of the firing method.

In an electrical energy supply system (power grid) the generated powerhas to be constantly in equilibrium with the consumer power. Changes tothe consumer load or power plant fault impair this equilibrium and causefrequency deviations in the grid, to which the machines involved in theprimary control or the primary requirement, react. Because of itscontrol behavior, the primary control or the equivalent primaryrequirement guarantees the restoration of the equilibrium betweengenerated and consumed power within a few seconds, while the frequencyis held within the permissible limit values. In the power grid, thereare quasi-stationary deviations (with respect to the target values) bothof the frequency Δ f as well as the transfer power Δ Pi between theindividual control zones following the balancing of a sudden change ofthe consumed or generated power through the primary control or theprimary requirement. In this connection, the secondary control orsecondary requirement becomes functional whose objective it is to returnthe frequency to its target value and the transfer outputs to the agreedvalues and thus to have the entire activated primary control outputagain available as reserve.

FIG. 1 shows the interpretation of the primary and secondary control orprimary and secondary control output or primary and secondaryrequirement (interpretation of primary and secondary response values) ofthe British Power Grid Regulations (Grid Code UK) that has to occur upona frequency deviation (frequency change) of −0.5 Hz from the targetfrequency of the power grid. The diagram of FIG. 1 shows that a powerplant connected to the power grid according to the primary control P hasto react within a time span T_(Sp) of 10 seconds with a plant responseand thus increase the power plant output. The amount of the outputincrease within this time span T_(Sp) is dependent on the load rangewith which the power plant happens to be operated at the time of thedrop in frequency. The British Power Grid Regulations determine forexample with a fixed required minimum load (minimum generation) of 65%of the nominal output (RC) (registered capacity) of the power plant thatwith this part load the power plant output has to be increased withinthe 10 seconds by 10% (percentage A_(p)) of the nominal output orcapacity RC of the power plant (see FIG. 2). According to FIG. 2 (theabscissa shows the load range (in percent of the RC) of the power plant,the ordinate shows the primary or secondary control ranges (in percentof the RC)) the increase by 10% of the rated capacity RC of the powerplant has to be guaranteed between the part load range of 65 to 80% ofthe nominal power plant output RC. Between the part load range of 80 to100% of the nominal power plant output RC the power increase decreaseslinearly from 10% to 0.

In the event of the frequency being exceeded or a reduction of theprimary and/or secondary requirements of the power grid operator withrespect to the current output into the grid it is provided according toFIG. 2 to lower the power plant output in the part load range between95% and 70% of the rated power of the power plant by 10% of the ratedpower RC of the power plant within the 10 seconds. Between the part loadranges of 70% to 65% of the nominal power plant output RC the outputreduction decreases linearly from 10% to approximately 6.5 and between100% and the part load range of 95%, the power reduction increaseslinearly from approximately 5% to 10%. FIG. 2 additionally shows theminimum load (minimum generation MG) of the power plant required by theBritish Power Grid, which is at 65% of the nominal power plant output.

FIG. 3 exemplarily shows how these requirements raised by the BritishPower Grid Regulations can be satisfied. To this end, the furnace 1 ofthe power plant according to the invention which is not shown isexemplarily designed with four coal grinding plants 2.1, 2.2, 2.3, 2.4,all of which directly fire the firing box of the power plant which isnot shown (direct firing system), wherein at least one of the coalgrinding plants 2.1, 2.2, 2.3, 2.4 is designed in such a manner that itcan be used to fire the firing box indirectly (indirect firing system)instead of directly, i.e. that at least one of the coal grinding plants2.1, 2.2, 2.3, 2.4 in addition to the direct firing system isadditionally designed with an indirect firing system.

Directly fired or a direct firing system means to say that the coalreduced in the coal grinding plant or coal mill 2.1, 2.2, 2.3, 2.4 isdirectly fed to the firing box by means of a carrier gas or support airvia coal dust lines 3.1, 3.2, 3.3, 3.4 and fired therein. Here,according to FIG. 3, a burner level each can be serviced by each coalgrinding plant 2.1, 2.2, 2.3, 2.4 and the coal dust lines 3.1, 3.2, 3.3,3.4 originating from the respective coal grinding plants 2.1, 2.2, 2.3,2.4 each service the burners which are not shown in the respectivecorners or side walls of the generally rectangular firing boxes of thecoal-fired power plant.

Indirectly fired or an indirect firing system means to say that the coalreduced or ground in the coal grinding plant or coal mill 2.1, 2.2, 2.3,2.4 is discharged via coal dust lines 3.1, 3.2, 3.3, 3.4 and initiallyconducted in the direction of the firing box, but then, via a coal dustswitch 6 each arranged in the coal dust line 3.1, 3.2, 3.3, 3.4 and viastorage lines 7.1, 7.2, 7.3, 7.4 is fed to a separator 4 common to allstorage lines. In the separator 4, the coal dust is separated from thecarrier gas or support air and via a connecting line 8 fed to a silo 5and stored therein. Via feed lines 9.1, 9.2, 9.3, 9.4 and regulatedapportioning organs 10 arranged in these feed lines the coal dust can beextracted from the silo 5 and via a charging device 15 each and afurther coal dust switch 13 fed to the coal dust lines 3.1, 3.2, 3.3,3.4 downstream of the first coal dust switches 6 in order to be conveyedinto the firing box by these. For conveying the coal dust extracted fromthe silo 5 into the firing box the charging devices 15 arranged in thefeed lines 9.1, 9.2, 9.3, 9.4 downstream of the apportioning organs 10are supplied with a conveying gas, for example air, via a conveying gasline 11, which air is supplied by a conveying gas blower 12. Thecharging device 15 can for example be an injector, a feeder shoe, a dustpump or the like.

The carrier gas or support air separated in the separator 4 isdischarged via a carrier gas discharge line 14 and fed into theatmosphere, while it is one more time cleaned before that in a dustseparating system. Instead of into the atmosphere, the carrier gas canalso be conducted into the firing box or the smoke gas drafts of thecoal-fired power plant connected downstream of the firing box and freedof dust in the existing dust separating system (e.g. e-filter, hosefilter of the like) of the power plant system.

Deviating from FIG. 3, each of the storage lines 7.1, 7.2, 7.3, 7.4 caneach have its own separator 4 and its own silo 5 connected downstream,from which the respective feed lines 9.1, 9.2, 9.3, 9.4 then originate.

In normal operation of the power plant, the coal grinding plants 2.1,2.2, 2.3 of the furnace 1 according to FIG. 3 work in such a manner thatthe coal dust ground therein is directly fed to the firing box forfiring via the respective carbon dust lines 3.1, 3.2, 3.3, 3.4. In thecase of the coal grinding plant 2.4, which exemplarily (instead of thegrinding plant 2.4, it can also be any other grinding plant) is designedwith an indirect firing system in addition to the direct firing system,the respective coal dust switches 6 and 13 arranged in the coal dustlines 3.1, 3.2, 3.3, 3.4 are each set in such a manner that the coaldust ground in the coal grinding plant 2.4 is not directly fed to thefiring box, but to the firing box by way of the silo 5. To this end, theapportioning organs 10 arranged in the feed lines 9.1, 9.2, 9.3, 9.4 andcharging devices 15 are in operation and conveying gas is provided tothe charging devices 15 through the conveying gas line 11 and theconveying gas blower 12. In the charging devices 15, the conveying gaspicks up the respective coal dust apportioned by the apportioning organs10 and conveys it into the firing box. The operation of the grindingplant 2.4 is such that as a rule, at the start of the operation, thegrinding output of the grinding plant 2.4 compared with the grindingoutput of the grinding plants 2.1, 2.2, 2.3 or compared with the currentrequirement of the grinding output of the grinding plant 2.4 or comparedwith the present actual output of the grinding plant 2.4 is increased inorder to half fill the volume of the silo 5 having a storage volumeV_(Sp) with the excess offer of ground fuel. Following completed fillingof the silo 5 the grinding output of the grinding plant 2.4 is adaptedto those of the grinding plant 2.1, 2.2, 2.3 or the current requirementof the grinding output of the grinding plant 2.4. With the exception ofthe filling operation of the silo 5, the discharge or conveying outputof the apportioning organs 10 corresponds to the quantity-based grindingoutput of the grinding plant 2.4, i.e. after the filling operation, thequantity of coal dust as produced by the grinding plant 2.4 isdischarged from the silo 5 and conveyed into the silo 5, while minutelosses in the separator 4 are taken into account.

In the case of a frequency change or a frequency drop or a frequencyundershot by for example 0.5 Hz of the power grid the block outputcontrol of the coal-fired power plant is influenced via the grid controlof the power grid or its primary and/or secondary requirements of thepower grid operator with respect to the current output into the grid,which substantially increases the quantity of the coal dust dischargedby the apportioning organs 10 from the silo 5 and indirectly fed to thefiring box relative to the present actual output or relative to the coaldust quantities in each case supplied by the coal grinding plants 2.1,2.2, 2.3. During this, the coal dust stored and stocked in the silo 5for these purposes can be introduced into the firing box for firing in avery short time and thereby, on the part of the furnace, a substantialcontribution can be made for improving the dynamic behavior of thecoal-fired power plant. The present actual output designates the outputor the part load with which the coal-fired power plant is currentlyoperated and on which the fuel quantity fed in to the firing box andthus also the respective throughput rate of the individual coal grindingplants 2.1, 2.2, 2.3, 2.4 is dependent.

In the event of a frequency being exceeded for example by 0.5 Hz of thepower grid the block output control of the coal-fired power plant isinfluenced via the grid control of the power grid or its primary and/orsecondary requirements of the power grid operator with respect to thecurrent output into the grid, which substantially reduces the quantityof the coal dust discharged by the apportioning organs 10 from the silo5 and indirectly fed to the combustion chamber compared with the presentactual output or compared with the coal dust quantities supplied in eachcase by the coal grinding plants 2.1, 2.2, 2.3 and thus, as with theincrease of the coal dust quantity, a substantial contribution is madeby the furnace to the improvement of the dynamic behavior of thecoal-fired power plant. Here, coal dust provided by the grinding plant2.4 during this process and which is not necessary, i.e. excess, isbuffer-stored in the silo 5.

For realizing the improvement of the dynamic behavior of a coal-firedpower plant the silo 5 connected downstream of the coal grinding plant2.4 is dimensioned and designed with a receiving capacity or a storagevolume V_(Sp) for the coal dust to be stored. However, additional coalgrinding plants of the exemplary four coal grinding plants 2.1, 2.2,2.3, 2.4 in FIG. 3 can each be designed with an indirect firing systemand thus with a silo 5 for the storage of coal dust. If for example two,three or all four coal grinding plants 2.1, 2.2, 2.3, 2.4 areadditionally designed with an indirect furnace or an indirect firingsystem the entire storage volume or the receiving capacity V_(Sp) ofcoal dust can be divided over the existing number of silos 5 or thestorage volume V_(Sp) increased through the increased number of silos 5.Through the additional design of a plurality of grinding plants withindirect firing system and thus increased coal dust storage capacity inthe silos 5 the dynamics of the fuel apportioning of the coal-firedpower plant can be further improved if required. Through thisimprovement of the dynamics of the fuel the primary and secondaryreserve of the coal-fired power plant can also be improved or increased.

The storage volume V_(Sp) of the silo 5 is dimensioned in such a mannerthat upon normal operation, i.e. with stationary state, the storagevolume V_(Sp) of the silo 5 is filled to about half and thereby hasstored sufficient coal dust in order to be able to introduce anincreased coal dust quantity into the firing box in the event of afrequency drop or a primary and/or secondary requirement of the powergrid operator with respect to the current output into the grid, i.e. ofan instationary state, in order to improve the dynamic behavior of thepower plant. On the other hand, the silo 5 still has to have sufficientstorage capacity in order to be able to introduce a reduced coal dustquantity into the firing box in the event of a frequency being exceededor a primary and/or secondary requirement of the power grid operatorwith respect to the current output into the grid, i.e. in turn of aninstationary state, and thereby receive or store the excess coal dustquantity produced by the grinding plant 2.4 during the instationarystate in the silo 5.

In addition to the silo or the silos 5 the apportioning organs 10, thecharging devices 15 and the coal dust lines (feeding lines 9.1, 9.2,9.3, 9.4 and coal dust lines 3.1, 3.2, 3.3, 3.4) can be suitablydesigned dimensionally downstream of the silo or of the silos 5 as faras to the firing box in order to be able to conduct and feed to thefiring box the required fuel quantities in the short time required. Theconveying gas or support air required for this purpose is supplied in acontrolled manner through the conveying gas line 11 and by means of theconveying gas blower 12.

FIG. 4 schematically shows the dynamic behavior of a direct and anindirect furnace or of a direct as well as an indirect firing system ofa coal-fired power plant. While the increase of the boiler output fromL₀ to L₁ with the direct furnace starting out from t₀ takes the time t₂,the increase of the same boiler output with the indirect furnacestarting out from t₀ only requires the time t₁ and thus comessubstantially closer to an ideal, rapid increase within a time t₀ (stepresponse). Through the method according to the invention or the assemblyaccording to the invention of designing at least one of the coalgrinding plants 2.1, 2.2, 2.3, 2.4 in addition to the direct furnacewith an indirect furnace and operating said furnace as indirect furnaceand upon a frequency change in the power grid or a primary and/orsecondary requirement of the power grid operator with respect to thecurrent output into the grid of increasing or reducing the quantity ofthe coal dust discharged from the silo 5 and indirectly fed to thefiring box compared with the indirectly supplied coal dust quantity uponstable grid frequency, the dynamic behavior of the furnace according toFIG. 4 and thus also the plant response behavior, i.e. the dynamicbehavior of the coal-fired power plant can be substantially improved.The increase of the boiler output from L₀ to L₁ constitutes a percentageA_(p) of the nominal power plant output RC, for example an increase by10% of the nominal power plant output RC.

In the event of a maintenance or a failure of an apportioning organ 10or of a charging device 15 of the indirect firing system on the coalgrinding plant 2.4 the operation of the coal grinding plant 2.4 asdirect firing system can be continued in that the coal dust switches 6and 13 are reset and the coal dust through the coal dust lines 3.1, 3.2,3.3, 3.4 is directly fed to the firing box and the silo 5 as well as theapportioning organs 10 and the charging device 15 are thus bypassed. Iffurther coal grinding plants 2.1, 2.2, 2.3 are additionally designedwith an indirect firing system, one or a plurality of coal grindingplants can be reset by means of resetting of the coal dust switches 6and 13 to the operation as indirect firing system and thus temporarilyreplace the indirect firing system of the coal grinding plant 2.4currently undergoing maintenance.

Obviously, with the method according to the invention or the assemblyaccording to the invention regarding the primary and secondary controlor the primary and secondary requirement and from this the plantresponse behavior or with respect to the improved dynamic behavior of acoal-fired power plant not only the exemplarily mentioned British PowerGrid Regulations and their requirements can be maintained or satisfied,but also further national or international regulations requiring a rapidor improved dynamic behavior of the coal-fired power plant. To this end,if required, merely the storage volume V_(Sp) of the silo or silos 5 andthe throughput rates of the apportioning organs 10 and/or of thecharging devices 15 and/or of the conveying gas blower 12 have to beadapted to the regulations.

LIST OF REFERENCE NUMBERS

-   1 Furnace-   2.1 Coal grinding plant-   2.2 Coal grinding plant-   2.3 Coal grinding plant-   2.2 Coal grinding plant-   3.1 Coal dust line-   3.2 Coal dust line-   3.3 Coal dust line-   3.4 Coal dust line-   4 Separator-   5 Silo-   6 Coal dust switch-   7.1 Storage line-   7.2 Storage line-   7.3 Storage line-   7.4 Storage line-   8 Connecting line-   9.1 Feed line-   9.2 Feed line-   9.3 Feed line-   9.4 Feed line-   10 Apportioning organ-   11 Conveying gas line-   12 Conveying gas blower-   13 Coal dust switch-   14 Carrier gas discharge line-   15 Charging device

1. A method for improving the dynamic behavior of a coal-fired powerplant for primary and/or secondary requirements of the power gridoperator with respect to the current output into the grid, wherein thepower plant has a nominal output (RC) and is operated with a furnacecomprising at least one firing box for the firing of the fuel, at leasttwo coal grinding plants for the grinding of the fuel having a directfuel system, wherein at least one of these coal grinding plantscomprises an additional indirect firing system and the coal dust isindirectly fed to the firing box via the indirect firing system havingat least one silo and apportioning organs and the further coal grindingplant(s) directly feeds the coal dust to the firing box via the directfiring system, and wherein upon an increase of the primary and/orsecondary requirements of the power grid operator with respect to thecurrent output into the grid the coal dust quantity indirectly fed invia silo and apportioning organs compared with the present actual outputor compared with the coal dust quantity fed in through each of the coalgrinding plant(s) is increased and in the process coal dust stocked inthe silo is withdrawn and introduced into the firing box, and whereinupon a reduction of the primary and/or secondary requirements of thepower grid operator with respect to the current output into the grid thecoal dust quantity indirectly fed in via silo and apportioning organs isreduced compared with the present actual output or compared with thecoal dust quantity fed in through each of the coal grinding plant(s) isreduced and in the process coal dust excessively produced by thegrinding plant stored in the silo.
 2. The method as claimed in claim 1,characterized in that the silo has a storage volume (V_(Sp)) and innormal operation of the indirect firing system with regard to volume isfilled to about half with coal dust for stocking and use upon anincrease of the primary and/or secondary requirements of the power gridoperator with respect to the power output into the grid and theremaining storage volume is used for receiving and storing the excessproduced coal dust upon reduction of the primary and/or secondaryrequirements of the power grid operator with respect to the currentoutput into the grid.
 3. The method as claimed in claim 1, characterizedin that the increase or reduction of the indirectly fed-in coal dustquantity is effected through controlled increase or reduction of thethroughput rate of the apportioning organs.
 4. The method as claimed inclaim 1, characterized in that upon an increase or a reduction of theindirectly fed-in coal dust quantity the volumetric flow of theconveying gas blower is increased or reduced in a controlled manner. 5.The method as claimed in claim 3, characterized in that the increase orthe reduction of the throughput rate of the apportioning organs and/orthe increase or reduction of the volumetric flow of the conveying gasblower is brought about by the block output control of the coal-firedpower plant influenced by the requirements of the power grid.
 6. Themethod as claimed in claim 1, characterized in that the primaryrequirement is triggered through a remote-controlled signal.
 7. Themethod as claimed in claim 1, characterized in that the secondaryrequirement is triggered through a remote-controlled signal.
 8. Themethod as claimed in claim 1, characterized in that the secondaryrequirement is triggered through written or oral instruction to theoperating personnel of the power plant.
 9. An assembly for improving thedynamic behavior of a coal-fired power plant for primary and/orsecondary requirements of the power grid operator with respect to thecurrent output into the grid, wherein the power plant has a nominaloutput (RC) and is designed with a furnace (1) which substantiallycomprises at least one firing box for the firing of the fuel, at leasttwo coal grinding plants (2.1, 2.2) for the grinding of the fuelcomprising a direct firing system, wherein at least one of these coalgrinding plants (2.1, 2.2) comprises an additional indirect firingsystem and the coal dust can be indirectly fed to the firing box via theindirect firing system having at least one silo (5) and apportioningorgans (10) and with the further coal grinding plant(s) (2.1, 2.2) thefiring box can be directly fed with coal dust via the direct firingsystem, and wherein upon an increase of the primary and/or secondaryrequirements of the power grid operator with respect to the currentoutput into the grid the coal dust quantity that can be indirectly fedin via silo (5) and apportioning organs (10) compared to the presentactual output or compared to the coal dust quantity that can be fed inby the coal grinding plant(s) (2.1, 2.2) in each case can be increasedand in the process coal dust stocked in the silo (5) can be withdrawnand introduced into the firing box and wherein upon a reduction of theprimary and/or secondary requirements of the power grid operator withrespect to the current output into the grid the coal dust quantity thatcan be indirectly fed in via silo (5) and apportioning organs (10)compared with the present actual output or compared with the coal dustquantity that can be fed in through the coal grinding plant(s) (2.1,2.2) in each case can be reduced and in the process excess coal dustproduced by the grinding plant (2.1, 2.2) can be stored in the silo (5).